It is known to polymerize one or more olefins in the gaseous phase at a pressure which is higher than atmospheric pressure in a reactor with a fluidized bed and a vertical side wall, where polymer particles being formed are kept in the fluidized state above a fluidization grid by virtue of a reaction gas mixture containing the olefin(s) to be polymerized and travelling according to an upward stream. The polymer thus manufactured in powder form is generally drawn off from the reactor by at least one side discharge conduit situated along the vertical wall of the reactor above the fluidization grid and is then subjected to a decompression and degassing stage. The reaction gas mixture leaving via the top of the fluidized-bed reactor is returned to the base of the latter under the fluidization grid through the intermediacy of an external circulation conduit provided with a compressor. While being returned, the reaction gas mixture is generally cooled with the aid of at least one heat exchanger provided in the external circulation conduit so as to remove the heat produced by the polymerization reaction. The polymerization is carried out in the presence of a catalyst or of a catalyst system introduced into the fluidized bed. High-activity catalysts and catalyst systems, which have been known already for a number of years, are capable of producing large quantities of polymer in a relatively short time, thus avoiding a stage of removal of the catalyst residues in the polymer.
When the polymer is drawn from the reactor, it is found to be accompanied by the reaction gas mixture present under pressure in the reactor. It has been observed that the proportion of the reaction gas mixture accompanying the polymer which is drawn off is generally high. This makes it necessary to provide large-sized devices for decompressing and degassing the polymer which is drawn off, and an appropriate and costly device for recovering and recompressing the reaction gas mixture drawn off with the polymer and for recycling most of this gas mixture into the polymerization reactor. Such devices generally include a lock hopper for polymer recovery, connected to the side wall of the fluidized-bed reactor by a draw-off conduit fitted with an isolation valve. The lock hopper may also be connected to a decompression and degassing chamber by a discharge conduit, itself fitted with an isolation valve. In most cases the draw-off conduit leaves the side wall of the reactor in a direction that is perpendicular to the said wall, that is to say in a horizontal plane. It then reaches a lock hopper either while still remaining in the same horizontal plane, as described in U.S. Pat. No. 4 003 712, French Patent No. 2 642 429 or European Patent No. 188 125, or while descending vertically after having formed an elbow, as described in European Patent No. 71 430. The draw-off conduit may also leave the side wall of the reactor while being directed downwards and while forming a relatively small angle with a horizontal plane, for example an angle of 18.degree. as shown in FIG. 1 of French Patent No. 2 599 991, and next reach a lock hopper while descending vertically after having formed an elbow. It has been observed that, in all cases, the proportion of the reaction gas mixture accompanying the polymer which is drawn off is relatively high and results in the above-mentioned disadvantages. To solve this problem, European Patent No. 71 430 proposes to use a separator for polymer recovery comprising a vent conduit system comprising a valve and connecting the said separator to the fluidized-bed reactor at a point situated above the draw-off conduit near to the top of the fluidized bed for venting back to the reactor the gas separated from the polymer in the separator. The same proposal is disclosed in French Patent No. 2 164 500 for drawing off a catalyst from a fluidized bed reactor in a fluidized catalyst cracking (FCC) process. A mixture of fluid and catalyst is drawn from the bottom of the reactor into a separator which is located below the level of the reactor and wherein the fluid is separated from the catalyst and is recycled from the separator into the upper part of the reactor near the top of the fluidized bed via a vent conduit provided with a valve.